Studies of the interaction of myxoviruses and other viruses with erythrocytes (hemaglutination, hemolysis, hemadsorption) will continue based on 1) the further chemical and immunochemical characterization of membrane proteins, particularly native and degraded glycoproteins (virus receptor substances, VRS) from human, rhesus and murine erythrocytes; and 2) the isolation of paramyxovirus (Sendai) envelope subunits, particularly neuraminidase, and the development of corresponding monospecific antisera. A similar immunochemical approach will be directed to the study of red cell fusion and polykaryon formation in cultured cells by exogenously applied inactive Sendai virus; and to the morphogenesis of viruses which fuse cells during growth in cell monolayers. In the latter category, respiratory syncytial (RS) virus and pneumonia virus of mice (PVM) will be examined by light and electron microscopy with labeled antibody (fluorescein, ferritin, peroxidase). PVM reacts with murine eyrthrocytes, RS with none. We have murine erythrocyte components and corresponding antisera with which to examine the PVM receptor system which differs from that of myxoviruses.